The current invention concerns nuclear reactors. More specifically, the current invention concerns an apparatus and method for replacing the internal components, such as the lower internals, of a nuclear reactor.
In a nuclear reactor, the internal components typically consist of upper and lower internals. The upper internals include a control rod guide tube assembly, support columns, conduits for instrumentation which enters the reactor vessel through the closure head, and a fuel assembly alignment structure, referred to as an upper core plate. The lower internals include a core support structure, referred to as a core barrel, and conduits for instrumentation which enters the reactor vessel at its bottom. After many years of service, it is often desirable to replace a portion of the reactor internals, especially the lower internals, which are in close proximity to the core. Such replacement not only has the potential to add years to the reactor life by substituting new components for those which have been exposed to the vibration, radiation and thermal stress associated with operation of the reactor, it also allows the introduction of upgraded equipment developed after the initial assembly of the reactor in the factory. For instance, better shielding designs can be incorporated in the core baffle region so that more fast neutrons are reflected back into the core resulting in both fuel efficiencies and an extended reactor vessel life. Despite the advantages of replacing the lower internals, in the past, such replacement during an outage for re-fueling was thought impractical. This was so because the new lower internals must be precisely aligned with respect to the existing upper internals and pressure vessel in order to ensure proper operation of the reactor. According to the prior art approach, such precise alignment would have required extensive on-site work to customize the new lower internals to the existing upper internals and the pressure vessel. This on-site work resulted in (i) an unacceptable increase in reactor down-time over that associated with a re-fueling operation and (ii) excessive exposure of maintenance personnel to radiation.
Accordingly, it would be desirable to provide an apparatus and method which enabled replacement of the lower internal components in a nuclear reactor in a relatively short period of time and with only minimum exposure of personnel to radiation and yet which resulted in accurate alignment of the new components to the existing ones.